The present invention relates to a biaxially stretched polyparaphenylene sulfide film which is excellent in slip properties and oil-impregnation property, and more in detail, the present invention relates to a biaxially stretched polyparaphenylene sulfide film comprising a resin composition prepared by mixing from 0.3 to 15 parts by weight of poly(4-methylpentene-1) with 100 parts by weight of a resin of polyparaphenylene sulfide series.
As the industrial films (engineering plastic film), although polyethylene terephthalate films, polypropylene films, polyimide films, etc. have been exemplified, they are not sufficient as the film which can be formed by melt-processing, is excellent in various properties such as heat-resistance and is cheap in the production cost.
On the other hand, the resin of polyphenylene sulfide series (hereinafter, referred to as PPS) is excellent in heat-resistance and chemical-resistance, and has been used for various uses as a material for incombustible films.
In the case where PPS films are used as a capacitor film or a base film for magnetic tape, by adding an inorganic filler, for instance, silica, carbon, glass, kaoline, calcium carbonate and calcium phosphate to PPS resin, the oil-impregnation property of the film is improved and a PPS film in which the running property of the tape is improved is produced.
However, in the case of the industrial production of the film of which the oil-impregnation property and the running property of the tape have been improved by adding the inorganic filler to PPS resin, various technical problems are caused.
For instance, the thus added inorganic filler forms protuberances on the surface of the thus produced film. Furthermore, since the affinity between the inorganic filler and the PPS resin is poor and the inorganic filler itself coagulates in the PPS resin, the inorganic filler comes off from the film to be white powder during the running in the production of the film and during the use of the film as a magnetic film or during the production of a capacitor, extremely inconvenient situations are caused in the industrial processes for producing the goods in the case of using the inorganic filler.
Further, although the content of the inorganic filler in the base film for the magnetic tape is relatively small to PPS resin, since it has been well known that the performance of the film as the magnetic tape is lowered in the case where the unevenness of the surface of the film caused by the inorganic filler is too severe, the solution of the problem how to make the surface of the film flat and smooth while maintaining the properties of the film, such as the running property has been demanded.
On mixing the inorganic filler with PPS resin, there is a tendency that the electric specificity of the film, namely the electrical breakdown strength thereof is reduced. Furthermore, there may be cases where the dielectric loss (tan .delta.) of the film is raised by the water-absorption of the thus mixed inorganic filler. Since the dielectric loss is raised by using a relatively large amount of the inorganic filler, the increase of the dielectric loss causes the self-generation of heat in the capacitor film resulting in the destruction of the capacitor and therefore, the solution of the problem of suppressing the increase of the dielectric loss has also been demanded.
On the other hand, the PPS film prepared without adding the inorganic filler has the defects of poor in the running property and also poor in the oil-impregnation property.
Accordingly, an offer of a biaxially stretched PPS film, which is excellent in the slipping properties, the running property and the oil-impregnation property and does not contain the inorganic filler, has been earnestly desired.
As a result of the present inventors' studies, it has been found out by the present inventors that the biaxially stretched film produced from the resin composition prepared by blending poly(4-methylpentene-1) with a resin of polyparaphenylene sulfide series has the excellent running property and the excellent oil-impregnation property, is excellent in the flatness and is low in dielectric loss, and on the basis of the finding, the present invention has been attained.